Channel quality feedback mechanism and method

ABSTRACT

Methods and apparatus are presented for improving the feedback of channel information to a serving base station, which allows a reduction in the reverse link load while allowing the base station to improve the forward link data throughput. Over a channel quality indicator channel, three subchannels are generated; the re-synch subchannel, the differential feedback subchannel, and the transition indicator subchannel. The information carried on each subchannel can be used separately or together by a base station to selectively update internal registers storing channel conditions. The channel conditions are used to determine transmission formats, power levels, and data rates of forward link transmissions.

BACKGROUND

[0001] 1. Field

[0002] The present invention relates generally to communications, andmore specifically, to improving the feedback of channel information,which can be used to improve the scheduling and rate control of trafficover a wireless communication system.

[0003] 2. Background

[0004] The field of wireless communications has many applicationsincluding, e.g., cordless telephones, paging, wireless local loops,personal digital assistants (PDAs), Internet telephony, and satellitecommunication systems. A particularly important application is cellulartelephone systems for mobile subscribers. As used herein, the term“cellular” system encompasses both cellular and personal communicationservices (PCS) frequencies. Various over-the-air interfaces have beendeveloped for such cellular telephone systems including, e.g., frequencydivision multiple access (FDMA), time division multiple access (TDMA),and code division multiple access (CDMA). In connection therewith,various domestic and international standards have been establishedincluding, e.g., Advanced Mobile Phone Service (AMPS), Global System forMobile (GSM), and Interim Standard 95 (IS-95). IS-95 and itsderivatives, IS-95A, IS-95B, ANSI J-STD-008 (often referred tocollectively herein as IS-95), and proposed high-data-rate systems arepromulgated by the Telecommunication Industry Association (TIA) andother well known standards bodies.

[0005] Cellular telephone systems configured in accordance with the useof the IS-95 standard employ CDMA signal processing techniques toprovide highly efficient and robust cellular telephone service.Exemplary cellular telephone systems configured substantially inaccordance with the use of the IS-95 standard are described in U.S. Pat.Nos. 5,103,459 and 4,901,307, which are assigned to the assignee of thepresent invention and incorporated by reference herein. An exemplarysystem utilizing CDMA techniques is the cdma2000 ITU-R RadioTransmission Technology (RTT) Candidate Submission (referred to hereinas cdma2000), issued by the TIA. The standard for cdma2000 is given inthe draft versions of IS-2000 and has been approved by the TIA and3GPP2. Another CDMA standard is the W-CDMA standard, as embodied in3^(rd) Generation Partnership Project “3GPP”, Document Nos. 3G TS25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214.

[0006] The telecommunication standards cited above are examples of onlysome of the various communication systems that can be implemented. Someof these various communication systems are configured so that remotestations can transmit information regarding the quality of thetransmission medium to a serving base station. This channel informationcan then be used by the serving base station to optimize the powerlevels, the transmission formats, and the timing of forward linktransmissions, and further, to control the power levels of reverse linktransmissions.

[0007] As used herein, “forward link” refers to the transmissionsdirected from a base station to a remote station and “reverse link”refers to transmissions directed from a remote station to a basestation. The forward link and the reverse link are uncorrelated, meaningthat observations of one do not facilitate the prediction of the other.However, for stationary and slow-moving remote stations, thecharacteristics of the forward link transmission path will be observedto be similar to the characteristics of the reverse link transmissionpath in a statistical sense.

[0008] Channel conditions of received forward link transmissions, suchas the carrier-to-interference (C/I) ratio, can be observed by a remotestation, which reports such information to a serving base station. Thebase station then uses this knowledge to schedule transmissions to theremote station selectively. For example, if the remote station reportsthe presence of a deep fade, the base station would refrain fromscheduling a transmission until the fading condition passes.Alternatively, the base station may decide to schedule a transmission,but at a high transmission power level in order to compensate for thefading condition. Alternatively, the base station may decide to alterthe data rate at which transmissions are sent, by transmitting data informats that can carry more information bits. For example, if thechannel conditions are bad, data can be transmitted in a transmissionformat with redundancies so that corrupted symbols are more likely to berecoverable. Hence, the data throughput is lower than if a transmissionformat without redundancies were used instead.

[0009] The base station can also use this channel information to balancethe power levels of all the remote stations within operating range, sothat reverse link transmissions arrive at the same power level. InCDMA-based systems, channelization between remote stations is producedby the use of pseudorandom codes, which allows a system to overlaymultiple signals on the same frequency. Hence, reverse link powercontrol is an essential operation of CDMA-based systems because excesstransmission power emitted from one remote station could “drown out”transmissions of its neighbors.

[0010] In communication systems that use feedback mechanisms todetermine the quality of the transmission media, channel conditions arecontinuously conveyed on the reverse link. This produces a large loadupon the system, consuming system resources that could otherwise beallocated to other functions. Hence, there is a need to reduce thereverse link load of unnecessary transmissions, which can occur when theremote stations transmit C/I information that have not changedsubstantially from the previous transmissions. However, the system muststill be able to detect and react to changing channel conditions in atimely manner. The embodiments described herein address these needs byproviding a mechanism for optimizing the transmission of channelinformation on the reverse link and for decoding such information at abase station.

SUMMARY

[0011] Methods and apparatus are presented herein to address the needsstated above. In one aspect, an apparatus is presented for schedulingforward link transmissions, the apparatus comprising: a memory element;and a processing element configured to execute a set of instructionsstored on the memory element, the set of instructions for: receiving afull channel quality value and a plurality of incremental channelquality values from a remote station, wherein the plurality ofincremental channel quality values are received sequentially; andselectively updating a register with a channel quality estimate, whereinthe channel quality estimate is based upon the full channel qualityvalue and the plurality of incremental channel quality values.

[0012] In another aspect, a method is presented for estimating forwardlink channel quality from a full channel quality value and a pluralityof incremental channel quality values, the method comprising: decodingthe full channel quality value over a plurality of slots; incrementallyupdating a channel state register with the plurality of incrementalchannel quality values, wherein each of the plurality of incrementalchannel quality values are received separately over each of theplurality of slots; and resetting the channel state register with thefull channel quality value when the full channel quality value is fullydecoded.

[0013] In another aspect, an apparatus is presented for transmittingchannel quality values over a feedback channel to a base station, theapparatus comprising: a re-synch subchannel generation system forgenerating full channel quality values; and a differential feedbacksubchannel generation system for generating a plurality of incrementalvalues, wherein the plurality of incremental values are multiplexed withthe full channel quality values.

[0014] In another aspect, a method is presented for transmitting channelinformation from a remote station to a base station, the methodcomprising: generating a full channel quality value; and generating anincremental channel quality value, wherein the incremental channelquality value is multiplexed with the full channel quality value.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a diagram of a wireless communication network.

[0016]FIG. 2A, FIG. 2B, and FIG. 2C are timelines that describe theinteractions between the re-synch subchannel and the differentialfeedback subchannel.

[0017]FIG. 3 is a functional block diagram of a remote station incommunication with a base station.

[0018]FIG. 4A, FIG. 4B, FIG. 4C, and FIG. 4D are timelines of differentimplementations of the re-synch subchannel and the differential feedbacksubchannel.

[0019]FIG. 4E is a table illustrating different values arising fromdifferent interpretations of the information received on the re-synchsubchannel and the differential feedback subchannel.

[0020]FIG. 5 is a graph illustrating an advantage of the “accumulate andadd” method when a deep fade occurs.

[0021]FIG. 6A and FIG. 6B are block diagrams of channel elements forgenerating re-synch subchannel, the differential feedback subchannel,and the transition indicator subchannel.

[0022]FIG. 7 is a graph illustrating an advantage of using the re-synchsubchannel and the differential feedback subchannel at quantizationlimits.

DETAILED DESCRIPTION

[0023] As illustrated in FIG. 1, a wireless communication network 10 maygenerally includes a plurality of mobile stations (also called remotestations or subscriber units or user equipment) 12 a-12 d, a pluralityof base stations (also called base station transceivers (BTSs) or NodeB). 14 a-14 c, a base station controller (BSC) (also called radionetwork controller or packet control function 16), a mobile switchingcenter (MSC) or switch 18, a packet data serving node (PDSN) orinternetworking function (IWF) 20, a public switched telephone network(PSTN) 22 (typically a telephone company), and an Internet Protocol (IP)network 24 (typically the Internet). For purposes of simplicity, fourmobile stations 12 a-12 d, three base stations 14 a-14 c, one BSC 16,one MSC 18, and one PDSN 20 are shown. It would be understood by thoseskilled in the art that there could be more or less number of mobilestations 12, base stations 14, BSCs 16, MSCs 18, and PDSNs 20.

[0024] In one embodiment the wireless communication network 10 is apacket data services network. The mobile stations 12 a-12 d may be anyof a number of different types of wireless communication device such asa portable phone, a cellular telephone that is connected to a laptopcomputer running IP-based, Web-browser applications, a cellulartelephone with associated hands-free car kits, a personal data assistant(PDA) running IP-based, Web-browser applications, a wirelesscommunication module incorporated into a portable computer, or a fixedlocation communication module such as might be found in a wireless localloop or meter reading system. In the most general embodiment, mobilestations may be any type of communication unit.

[0025] The mobile stations 12 a-12 d may advantageously be configured toperform one or more wireless packet data protocols such as described in,for example, the EIA/TIA/IS-707 standard. In a particular embodiment,the mobile stations 12 a-12 d generate IP packets destined for the IPnetwork 24 and encapsulate the IP packets into frames using apoint-to-point protocol (PPP).

[0026] In one embodiment the IP network 24 is coupled to the PDSN 20,the PDSN 20 is coupled to the MSC 18, the MSC is coupled to the BSC 16and the PSTN 22, and the BSC 16 is coupled to the base stations 14 a-14c via wirelines configured for transmission of voice and/or data packetsin accordance with any of several known protocols including, e.g., E1,T1, Asynchronous Transfer Mode (ATM), IP, PPP, Frame Relay, HDSL, ADSL,or xDSL. In an alternate embodiment, the BSC 16 can be coupled directlyto the PDSN 20.

[0027] During typical operation of the wireless communication network10, the base stations 14 a-14 c receive and demodulate sets of reversesignals from various mobile stations 12 a-12 d engaged in telephonecalls, Web browsing, or other data communications. Each reverse signalreceived by a given base station 14 a-14 c is processed within that basestation 14 a-14 c. Each base station 14 a-14 c may communicate with aplurality of mobile stations 12 a-12 d by modulating and transmittingsets of forward signals to the mobile stations 12 a-12 d. For example,as shown in FIG. 1, the base station 14 a communicates with first andsecond mobile stations 12 a, 12 b simultaneously, and the base station14 c communicates with third and fourth mobile stations 12 c, 12 dsimultaneously. The resulting packets are forwarded to the BSC 16, whichprovides call resource allocation and mobility management functionalityincluding the orchestration of soft handoffs of a call for a particularmobile station 12 a-12 d from one base station 14 a-14 c to another basestation 14 a-14 c. For example, a mobile station 12 c is communicatingwith two base stations 14 b, 14 c simultaneously. Eventually, when themobile station 12 c moves far enough away from one of the base stations14 c, the call will be handed off to the other base station 14 b.

[0028] If the transmission is a conventional telephone call, the BSC 16will route the received data to the MSC 18, which provides additionalrouting services for interface with the PSTN 22. If the transmission isa packet-based transmission such as a data call destined for the IPnetwork 24, the MSC 18 will route the data packets to the PDSN 20, whichwill send the packets to the IP network 24. Alternatively, the BSC 16will route the packets directly to the PDSN 20, which sends the packetsto the IP network 24.

[0029] In some communication systems, packets carrying data traffic aredivided into subpackets, which occupy slots of a transmission channel.For illustrative ease only, the nomenclature of a cdma2000 system isused hereafter. Such use is not intended to limit the implementation ofthe embodiments herein to cdma2000 systems. Implementations in othersystems, such as, e.g., WCDMA, can be accomplished without affecting thescope of the embodiments described herein.

[0030] The forward link from the base station to a remote stationoperating within the range of the base station can comprise a pluralityof channels. Some of the channels of the forward link can include, butare not limited to a pilot channel, synchronization channel, pagingchannel, quick paging channel, broadcast channel, power control channel,assignment channel, control channel, dedicated control channel, mediumaccess control (MAC) channel, fundamental channel, supplemental channel,supplemental code channel, and packet data channel. The reverse linkfrom a remote station to a base station also comprises a plurality ofchannels. Each channel carries different types of information to thetarget destination. Typically, voice traffic is carried on fundamentalchannels, and data traffic is carried on supplemental channels or packetdata channels. Supplemental channels are usually dedicated channels,while packet data channels usually carry signals that are designated fordifferent parties in a time and/or code-multiplexed manner.Alternatively, packet data channels are also described as sharedsupplemental channels. For the purposes of describing the embodimentsherein, the supplemental channels and the packet data channels aregenerically referred to as data traffic channels.

[0031] Voice traffic and data traffic are typically encoded, modulated,and spread before transmission on either the forward or reverse links.The encoding, modulation, and spreading can be implemented in a varietyof formats. In a CDMA system, the transmission format ultimately dependsupon the type of channel over which the voice traffic and data trafficare being transmitted and the condition of the channel, which can bedescribed in terms of fading and interference.

[0032] Predetermined transmit formats, which correspond to a combinationof various transmit parameters, can be used to simplify the choice oftransmission formats. In one embodiment, the transmission formatcorresponds to a combination of any or all of the following transmissionparameters: the modulation scheme used by the system, the number oforthogonal or quasi-orthogonal codes, an identification of theorthogonal or quasi-orthogonal codes, the data payload size in bits, theduration of the message frame, and/or details regarding the encodingscheme. Some examples of modulation schemes used within communicationsystems are the Quadrature Phase Shift Keying scheme (OPSK), 8-ary PhaseShift Keying scheme (8-PSK), and 16-ary Quadrature Amplitude Modulation(16-QAM). Some of the various encoding schemes that can be selectivelyimplemented are convolutional encoding schemes, which are implemented atvarious rates, or turbo coding, which comprises multiple encoding stepsseparated by interleaving steps.

[0033] Orthogonal and quasi-orthogonal codes, such as the Walsh codesequences, are used to channelize the information sent to each remotestation. In other words, Walsh code sequences are used on the forwardlink to allow the system to overlay multiple users, each assigned one orseveral different orthogonal or quasi-orthogonal codes, on the samefrequency during the same time duration.

[0034] A scheduling element in the base station is configured to controlthe transmission format of each packet, the rate of each packet, and theslot times over which each packet is to be transmitted to a remotestation. The terminology “packet” is used to describe system traffic.Packets can be divided into subpackets, which occupy slots of atransmission channel. “Slot” is used to describe a time duration of amessage frame. The use of such terminology is common in cdma2000systems, but the use of such terminology is not meant to limit theimplementation of the embodiments herein to cdma2000 systems.Implementation in other systems, such as, e.g. WCDMA, can beaccomplished without affecting the scope of the embodiments describedherein.

[0035] Scheduling is a vital component in attaining high data throughputin a packet-based system. In the cdma2000 system, the scheduling element(which is also referred to as a “scheduler” herein) controls the packingof payload into redundant and repetitious subpackets that can besoft-combined at a receiver, so that if a received subpacket iscorrupted, it can be combined with another corrupted subpacket todetermine the data payload within an acceptable frame error rate (FER).For example, if a remote station requests the transmission of data at76.8 kbps, but the base station knows that this transmission rate is notpossible at the requested time due to the condition of channel, thescheduler in the base station can control the packaging of the datapayload into multiple subpackets. The remote station will receivemultiple corrupted subpackets, but will still be likely to recover thedata payload by soft-combining the uncorrupted bits of the subpackets.Hence, the actual transmission rate of the bits can be different fromthe data throughput rate.

[0036] The scheduling element in the base station uses an open-loopalgorithm to adjust the data rate and scheduling of forward linktransmissions. The open-loop algorithm adjusts transmissions inaccordance with the varying channel conditions typically found in awireless environment. In general, a remote station measures the qualityof the forward link channel and transmits such information to the basestation. The base station uses the received channel conditions topredict the most efficient transmission format, rate, power level andtiming of the next packet transmission. In the cdma2000 1×EV-DV system,the remote stations can use a channel quality feedback channel (CQICH)to convey channel quality measurements of the best serving sector to thebase station. The channel quality may be measured in terms of acarrier-in-interference (C/I) ratio and is based upon received forwardlink signals. The C/I value is mapped onto a five-bit channel qualityindicator (CQI) symbol, wherein the fifth bit is reserved. Hence, theC/I value can have one of sixteen quantization values.

[0037] Since the remote station is not prescient, the remote stationtransmits the C/I values continuously, so that the base station is awareof the channel conditions if ever any packets need to be transmitted onthe forward link to that remote station. The continuous transmission of4-bit C/I values consumes the battery life of the remote station byoccupying hardware and software resources in the remote station.

[0038] In addition to the problems of battery life and reverse linkloading, there is also a problem of latency. Due to propagation andprocessing delays, the base station is scheduling transmissions usingoutdated information. If the typical propagation delay is 2.5 ms induration, which corresponds to a 2-slot delay in systems with 1.25 msslots, then the base station may be reacting to a situation that nolonger exists, or may fail to react in a timely manner to a newsituation.

[0039] For the above reasons, the communication network requires amechanism to convey information to the base station that allows the basestation to quickly reschedule transmissions due to sudden changes in thechannel environment. In addition, the aforementioned mechanism shouldreduce the drain on battery life of the remote station and the load onthe reverse link.

[0040] The embodiments described herein are directed to improving thefeedback mechanism for conveying channel information, such as C/l, fromthe remote station to the base station while reducing the load of thereverse link. By improving the feedback mechanism, the embodimentsimprove the ability of a base station to schedule transmissions and thedata rates of the transmissions in accordance with actual channelconditions. The embodiments are directed toward generating twosubchannels on the CQI channel in order to carry channel information. Itshould be noted that other channels could also be configured to carrythe subchannels described herein, but for the sake of expediency, theterminology of the CQI channel is used henceforth. The two subchannelsare referred to hereafter as the re-synch subchannel and thedifferential feedback subchannel.

[0041] In addition to improvements in the feedback mechanism at theremote station, improvements at the base station can also be implementedto optimize the interpretation of the channel information received fromthe remote station. A scheduling element in the base station can beconfigured to implement task functions in accordance with informationreceived from either subchannel or by selectively discarding informationreceived from either subchannel.

[0042] In a general description of the embodiments, full C/I values aretransmitted on the re-synch subchannel while incremental 1-bit valuesare transmitted over the differential feedback subchannel. Theincremental 1-bit values of 1 and 0 are mapped to +0.5 dB and −0.5 dB,but can be mapped to other values ±K as well, where K is a systemdefined step size.

[0043] Generation of Subchannels at a Remote Station

[0044] The values sent on the re-synch and differential feedbacksubchannels are determined based on the forward link C/I measurements.The value sent on the re-sync subchannel is obtained by quantizing themost recent C/I measurement. A one-bit value is sent on the differentialfeedback subchannel and is obtained by comparing the most recent C/Imeasurement with the contents of an internal register. The internalregister is updated based on past values sent on the re-synch anddifferential feedback subchannels, and represents the remote station'sbest estimate of the C/I value that the base station will decode.

[0045] In a first mode, channel elements can be placed within a remotestation to generate the re-synch subchannel and the differentialfeedback subchannel over the CQI channel (CQICH), wherein the re-synchsubchannel occupies one slot of an N-slot CQICH frame and thedifferential feedback subchannel occupies all slots of the N-slot CQICHframe, so that an incremental 1-bit value is transmitted in each slot.Hence, in at least one slot of the N-slot CQICH frame, both a full C/Ivalue and an incremental 1-bit value are transmitted to the basestation. This concurrent transmission is possible through the use oforthogonal or quasi-orthogonal spreading codes, or in an alternativeembodiment, by time interleaving the two subchannels in somepredetermined fashion. In an alternate first mode, the re-synchsubchannel and the differential feedback subchannel are not sent inparallel. Instead, the re-synch subchannel is transmitted over one slotand the system refrains from transmitting the differential feedbacksubchannel in that particular slot. FIG. 2A is a timeline illustratingthe transmission timing of the re-synch channel and the differentialfeedback subchannel operating in parallel in the first mode.

[0046] In a second mode, the channel elements are configured so that thetwo subchannels are generated with the re-synch subchannel operating ata reduced rate. The re-synch channel operates at a reduced rate when afull C/I value is spread over at least two slots of an N-slot CQICHframe. For example, the full C/I value may be transmitted at a reducedrate over 2, 4, 8, or 16 slots of a 16-slot CQICH frame. Thedifferential feedback subchannel occupies all of the slots of the N-slotCQICH frame. Hence, an incremental 1-bit value is transmitted in eachslot, in parallel to the re-synch subchannel. The remote station shouldtransmit the full C/I value at the reduced rate when the reverse link issuffering from unfavorable channel conditions. In one embodiment, thebase station determines the reverse link channel conditions andtransmits a control signal to the remote station, wherein the controlsignal informs the remote station as to whether the re-synch subchannelshould operate at a reduced rate or not. Alternatively, the remotestation can be programmed to make this determination independently.

[0047] In one implementation of the second mode, the two subchannelswork parallel at a reduced rate wherein a full C/I value is spread overall slots of a N-slot CQICH frame and each slot also carries anincremental 1-bit value. In an alternate second mode, the differentialfeedback subchannel occupies all of the slots of the N-slot frame exceptfor the first slot. In yet another alternate second mode, thedifferential feedback subchannel and the re-synch subchannel are notsent in parallel at all; the re-synch subchannel operates first over Mslots, and the differential feedback subchannel operates over the nextN-M slots of the N-slot frame. FIG. 2B and FIG. 2C are timelinesillustrating the transmission timing of the re-synch subchannel and thedifferential feedback subchannel operating in the second mode. Theinternal register of the remote station may be updated in the first,second or M^(th) slot, depending on which operating mode is in use.

[0048] In another embodiment, the full C/I value can also be sent atunscheduled slots, whenever the remote station determines that the C/Iestimate kept at the base station is out of synchronization. Thisembodiment requires that the base station continuously monitors theCQICH to determine whether an unscheduled full C/I value symbol ispresent or not.

[0049] In yet another embodiment, the full C/I value is only sent whenthe remote station determines that the C/I estimate kept at the basestation is out of synchronization. In this embodiment, the full C/Ivalue is not sent at regularly scheduled intervals.

[0050] Interpretation of Subchannel Information at a Base Station

[0051] A scheduling element in a base station can be configured tointerpret channel information received on the re-synch subchannel andthe differential feedback subchannel, wherein the channel informationfrom each subchannel is used to make transmission decisions that accountfor the state of the channel. The scheduling element can comprise aprocessing element coupled to a memory element, and is communicativelycoupled to the receiving subsystem and the transmission subsystem of thebase station.

[0052]FIG. 3 is a block diagram of some of the functional components ofa base station with a scheduling element. A remote station 300 transmitson the reverse link to a base station 310. At a receiving subsystem 312,the received transmissions are de-spread, demodulated and decoded. Ascheduler 314 receives a decoded C/I value and orchestrates theappropriate transmission formats, power levels, and data rates oftransmissions from the transmission subsystem 316 on the forward link.

[0053] At the remote station 300, a receiving subsystem 302 receives theforward link transmission and determines the forward link channelcharacteristics. A transmission subsystem 306, in which the channelelements described by FIGS. 6A and 6B are located, transmits suchforward link channel characteristics to the base station 310.

[0054] In the embodiments described herein, the scheduling element 314can be programmed to interpret the channel information received on there-synch subchannel together with the channel information received onthe differential feedback subchannel, or to interpret the channelinformation received on the re-synch subchannel separately from thechannel information received on the differential feedback subchannel.The scheduling element can also be configured to perform a method toalternate which subchannel will be used to update channel information.

[0055] When the remote station transmits the channel information usingthe first mode, a serving base station will receive the full C/I valueover one slot and incremental values over all slots of the frame. In oneembodiment, the scheduler can be programmed to reset internal registersthat store the current state of the channel, wherein the registers arereset with the full C/I value received over one slot of the re-synchsubchannel. The incremental values received over the different feedbacksubchannel are then added upon receipt to the full C/I value stored inthe register. In one aspect, the incremental value that is transmittedconcurrently over the slot with the full C/I value is intentionallydiscarded, since the full C/I value already accounts for thisincremental value.

[0056] When a remote station is operating in the second mode, a servingbase station will receive the full C/I value over multiple slots andincremental values over all slots of the frame. In one embodiment, theserving base station estimates the channel conditions at the time thatis scheduled for a packet transmission by accumulating the incrementalvalues received on the differential feedback subchannel from the secondslot to the M^(th) slot, where M is the number of slots over which thefull C/I value is spread out. This accumulated value is then added tothe full C/I value, which was received on the re-synch subchannel overthe M slots. In another embodiment, this “accumulate and add” method canbe performed concurrently with an independent action for “up-down” bits,which updates the C/I value stored in the register as directed by theincremental values. Hence, the register storing the current channelcondition information is updated each time an incremental value isreceived, and the register is then updated with the accumulated valueadded to the full C/I value.

[0057]FIGS. 4A, 4B, 4C and 4D are timelines describing the embodimentsabove. FIG. 4E is a table of C/I values stored in a register at a givenpoint in the timelines, using the embodiments described above. In thetimeline of FIG. 4A, the remote station is transmitting the re-synchsubchannel over a single slot of the CQICH frame and the differentialsubchannel over each slot of the CQICH frame. The base station isconfigured to update the register that stores the channel state suchthat parallel incremental values are discarded, i.e., the parallelincremental values are not used to update the register. Hence, at theinterval t₂-t₃, the channel state information stored in the register is4 dB, which is the full C/I value transmitted on the re-synch subchannelover interval t₁-t₂. The contribution of the differential feedbackchannel at interval t₁-t₂ is discarded.

[0058] In the timeline of FIG. 4B, the remote station is transmittingthe re-synch subchannel over multiple slots (4 slots in this example)and the differential subchannel over each slot of the CQICH frame.Again, the base station is configured to update the register that storesthe channel state such that parallel incremental values are discarded.Hence, at the interval t₁-t₅, the channel state information stored inthe register is 11 dB, which is the value of the different feedbacksubchannel over interval t₀-t₁ added to the stored value in theregister. The register is not updated with the value carried by there-synch subchannel until t₅, which is the instance when the re-synchC/I value has been fully received.

[0059] In the timeline of FIG. 4C, the remote station is transmittingthe re-synch subchannel over a single slot and the differentialsubchannel over each slot of the CQICH frame. In this example, one ofthe benefits of the embodiments described herein can be shown clearly.From interval t₀-t₁, the last value in the register is 10 dB. Frominterval t₁-t₂, the value in the register is 11 dB. If the re-synchsubchannel can be decoded correctly, then the register values over theintervals t₂-t₃ and t₃-t₄ would be the same as for the timeline in FIG.4A. However, if the re-synch subchannel cannot be decoded correctly,then the register values over the intervals t₂-t₃ and t₃-t₄ would be 10dB and 11 dB, respectively, rather than 4 dB and 5 dB. Even though, thefull C/I value is lost on the re-synch subchannel, the incrementalvalues received on the differential feedback subchannel can still beused to update the register. Hence, the differential feedback subchannelcan be used independently of the re-synch subchannel to update thechannel state information registers.

[0060] In the timeline of FIG. 4D, the remote station is transmittingthe re-synch subchannel over multiple slots (4 slots in this example)and the differential subchannel over each slot of the CQICH frame. Thebase station is configured to update the register that stores thechannel state, wherein the update accounts for the addition of parallelincremental values to the stored C/I re-synch value, as each incrementalvalue arrives at the base station.

[0061] In an alternative embodiment, the base station can be configuredto update the register that stores the channel state, wherein the updateincludes the accumulation of the parallel incremental values that isthen added to the stored C/I re-synch value. In particular, theaccumulate and add is performed using all incremental values except forthe incremental value transmitted in the first shared slot with the fullC/I value. Each parallel incremental value is added to the stored C/Ire-synch value as each arrives, and the aggregate of the incrementalvalues, except the first, is added to the newly received C/I value att₅.

[0062] The embodiments described above serve the practical purpose ofallowing the base station to more closely model the event of a deepfade. Rayleigh fading, also known as multipath interference, occurs whenmultiple copies of the same signal arrive at the receiver in adestructive manner. Substantial multipath interference can occur toproduce flat fading of the entire frequency bandwidth. If the remotestation is travelling in a rapidly changing environment, deep fadescould occur at scheduled transmission times. When such a circumstanceoccurs, the base station requires channel information that allows it toreschedule transmissions quickly and accurately. In the second mode, thebase station receives a reduced rate C/I value over more than one slot,but the base station can still compensate for the fade before the C/Ivalue is fully received over the multiple slots. FIG. 5 is a deep fadingcurve superimposed over a timeline that can be used to illustrate thepurpose of this embodiment.

[0063] At time t₀, a deep fading condition commences. Due to incrementalstep commands, the base station slowly models the fade, as shown by thedouble-dashed line. At time t₁, the remote station transmits a measuredC/I ratio at a reduced rate over multiple slots of the re-synchsubchannel. The remote station concurrently transmits incremental “up”commands on each slot to the base station. The base station startsdemodulating and decoding the C/I value on the re-synch subchannel.Since the 1-bit “up” command is relatively simple to demodulate anddecode, the base station can immediately start modeling the fade usingthe received up commands. At time t₂, wherein the C/I value is fullyprocessed, the base station resets its estimate of the channelconditions.

[0064] As shown by FIG. 5, without the use of the differential feedbackchannel, the base station would have continued to pursue a model ofchannel conditions that is sub-optimal. Rather than a model with apositive slope between the points t₁ and t₂, the model would have had anegative slope between points t₁ and t₂. Moreover, using the “accumulateand add” method, the base station would be able to estimate a highervalue of the channel state than the one already provided by the remotestation. Hence, the base station would have had a model that would beless accurate then the model created by the current embodiments.

[0065] The use of two subchannels as described above allows the basestation to react to the changing environment in which the remote stationis operating while minimizing the reverse link load. The reverse linkload is reduced because the majority of the slots will be carrying fewerinformation bits than continuous transmissions of full C/I values. Forexample, in the case of the second mode, one full C/I value is beingconveyed over all N slots of the CQICH frame, rather than thetransmission of N full C/I values over N slots.

[0066]FIG. 6A is a block diagram of channel elements that can implementthe modes described above in a cdma2000 1×EV-DV system. C/I ratio values601 are input into an encoder 602 at rate R=4/12 so that 12 binarysymbols are generated for each slot. The 12 binary symbols are spreadwith a Walsh code generated by a covering element 612. Covering element612 selects one of six allowed spreading Walsh sequences based on coversymbols 610 to indicate the index of the serving base station. Theoutput of the covering element 612 and the encoder 602 are combined byan adder 604 to form 96 binary symbols per slot. The output from theadder 604 is mapped in a mapping element 606 and then spread by a Walshspreading element 608 to generate the re-synch subchannel 600.Concurrently, incremental 1-bit values 621 are input into a repeater 622to form 96 binary symbols per slot. The repeated symbols are mapped in amapping element 624 and then spread by a Walsh spreading element 626 toform the differential feedback subchannel 620. The symbols sent on there-synch and the differential feedback subchannels are transmitted at arate of 1.2288 Mcps.

[0067]FIG. 6B is an alternate configuration wherein the concurrentincremental 1-bit values 621 are input into a repeater 622 to form 12binary symbols per slot. The rationale for this alternate configurationis discussed below in conjunction with the new transition indicatorsubchannel 630.

[0068] Base Station Index Indicator

[0069] The Walsh spreading introduced by covering element 612 of FIG. 6Aserves the purpose of indicating the best base station detected by theremote station, i.e. the base station with the highest forward link C/Ivalue, for the purposes of packet-based transmissions. It should benoted that the process of choosing a best base station for packet-basedtransmissions on a data traffic channel is different from the process ofchoosing the best base station for voice transmissions on a fundamentalchannel. For a voice transmission, a remote station that transitionsfrom the range of a first base station to a second base station willexchange voice traffic with both base stations at the same time in aprocess called soft handoff. Each base station operating within thenetwork is assigned a 20-bit identification value, and is ranked ingroups referred to as the active set, the candidate set, the neighborset, and the remaining set. Due to the variable nature of wirelessmedium, the ranking of base stations is a dynamic process.

[0070] The embodiments described herein are directed to data trafficchannels that exchange packets directed to individual base stations, dueto the nature of addressed packet data. In order to select the best basestation to serve the remote station, the remote station monitors forwardlink signals from all base stations in a designated “active set.” Asused herein, the “active set” for a packet-based transmission differsfrom the “active set” for a voice transmission.

[0071] Each member of the active set is assigned a different 3-bitindex, which is conveyed to the remote station from the serving basestation through signaling messages. The Walsh code to be used bycovering element 612 is selected based on the index corresponding to thebest base station in the active set. In FIG. 6A and FIG. 6B, the Walshspreading is applied only to the re-synch subchannel and not to thedifferential feedback subchannel. This embodiment has the advantage ofconserving Walsh-space, since only differential subchannel symbols aresent for a majority of the slots. Thus, the Walsh functions are usedinfrequently and are resources that can be directed to other purposes.In one aspect of this embodiment, the extra Walsh function is applied toa transition indicator subchannel, which is described below.

[0072] In another embodiment, the Walsh spreading is applied to both there-synch subchannel and the differential feedback subchannel, thus thebase station index indicator can be extracted from either.

[0073] In another embodiment, one of the Walsh functions is reserved forspreading the differential feedback subchannel symbols, while theremaining Walsh functions are used for spreading the re-synch subchannelsymbols to indicate the best base station index. This embodiment has thedisadvantage of reducing the number of available active set base stationindices by one. However, this embodiment provides for straightforward,concurrent use of the re-synch subchannel and the differential feedbacksubchannel, since they are spread with mutually orthogonal codes.

[0074] As a further advantage, when the new best base station is adifferent sector of the current serving base station, then the switchingof sectors can be immediate. The remote station can start sendingre-synch subchannel and differential feedback subchannel symbolscorresponding to the new best base station immediately.

[0075] When the new best base station is a sector of a different basestation, a transitional period for allowing a new forward link to be setup is desirable. In one embodiment, channel elements are configured togenerate a transition indicator subchannel. The transition indicatorsubchannel is set up so that a remote station can generate re-synchsubchannel symbols and differential feedback subchannel symbols thatcorrespond to the current base station's C/I value. This allows theremote station to utilize the existing forward link from the currentbase station. The transition indicator subchannel is shown in FIG. 6Aand FIG. 6B. Concurrent to the re-synch subchannel and the differentialfeedback subchannel, mismatch flag bits 631 are input into a repeater632 to form 48 binary symbols per slot in FIG. 6A and 12 binary symbolsper slot in the alternate configuration of FIG. 6B. The repeated symbolsare mapped in a mapping element 634 and then spread by a Walsh spreadingelement 636 to form the transition indicator subchannel 630. FIG. 6Aillustrates a transition indicator subchannel with Walsh function W₂₈³², while FIG. 6B illustrates a transition indicator subchannel withWalsh function W₂₈ ¹²⁸. The symbols sent on this subchannel aretransmitted at a rate of 1.2288 Mcps.

[0076] The transition indicator subchannel indicates the start of atransitional period to the current base station. The transitional periodis indicated by setting a bit in the transition indicator subchannel.The transition indicator subchannel may be transmitted in acode-multiplexed or time-multiplexed fashion. Code-multiplexing of thetransition indicator subchannel with other existing subchannels may beperformed through the use of a reserved Walsh spreading function.

[0077] In one embodiment, the transitional period is indicated by usinga Walsh spreading function that is the inverse of the Walsh spreadingfunction assigned to a base station in the non-transitional case. Asused herein, the inverse means using ‘0’ in place of ‘1’ and using ‘1’in place of ‘0’ in the Walsh sequence. This embodiment requires that theunion of the set of all code words generated by encoder 602 of FIG. 6Aor FIG. 6B and the set of inverses of all such code words forms acodebook that has satisfactory minimum distance properties. To achievethis, an appropriate encoder 602 must be used. One such possible encoderis obtained by puncturing the first four bits of a standard 16×16 Walshcode.

[0078] In one embodiment, all re-synch subchannel symbols aretransmitted at a reduced rate throughout the switching period to aidreliable detection of the switch from the current base station to a newbase station. To improve time diversity in fading channels, the reducedrate repetitions may be performed in non-consecutive slots. This aspectof the embodiment reduces C/I tracking performance by introducingadditional delays in the full C/I update, but increases the reliabilityof detecting the base station index indicator, which is of higherimportance.

[0079] Interpretation of Subchannel Information at Quantization Limits

[0080] As stated above, the C/I value is transmitted as 4 bits ofinformation; hence, there are only 16 possible values for the C/I value.The dynamic range of this quantization scheme is a system-definedparameter that can be altered without affecting the scope of theembodiments, i.e., more or less bits can be allocated for the dynamicrange of the C/I values. As one illustrative example, one quantizationscheme can be defined as having a minimum bit sequence value “0000” setequal to −15.5 dB and a maximum bit sequence value “1111” set equal to5.5 dB. A question arises as to the appropriate course of action atthese two extremes.

[0081] Using the above embodiments, if the channel conditions areextremely favorable at 8 dB over a long period of time, then the onlyvalue that the re-synch subchannel can transmit is 5.5 dB. The remotestation can attempt to compensate for this lack by transmittingincremental “up” bits to the base station. However, the base station isnot likely to take different actions for a channel condition of 5.5 dBversus 8 dB. Moreover, the decoding errors accumulated during the “abovethe limit” period will add to the tracking error even after the C/Ivalue drops below the maximum quantization level.

[0082] In one aspect of the embodiments above, the base station candeliberately ignore the values received on the differential feedbacksubchannel when a threshold C/I value is reached and a predeterminedpattern of transmissions on the differential feedback subchannel isdetected. In one example, a remote station determines that the conditionof the forward link is better than the maximum quantization value and sotransmits the maximum quantization value over the re-synch subchannel.In addition, the remote station deliberately transmits up bits to theserving base station throughout the duration that this favorable channelcondition exists. The transmission of up bits only is contrary to thepractice of transmitting up and down bits to track the slope of thefading curve. Referring back to FIG. 5, if the fading curve is above thethreshold amount at intervals t₁-t₃, then up bits would have been sentin interval t₁-t₂, and down bits would have been sent in interval t₂-t₃.However, using the embodiment described herein, only up bits would havebeen sent in the intervals t₁-t₂ and t₂-t₃.

[0083] The base station decodes the full C/I value on the re-synchsubchannel and determines that the full C/I value is equal to thethreshold value, which corresponds to the maximum value of the dynamicrange. If the base station then receives any up bits, the base stationis programmed to refrain from updating the registers that store thecurrent channel conditions until a full C/I value is received that isnot the threshold value. However, if the base station receives downbits, then the base station updates the registers accordingly.

[0084] In an additional embodiment, the remote station determines thatthe condition of the forward link is worse than the minimum quantizationvalue and so transmits the minimum quantization value over the re-synchsubchannel. In addition, the remote station deliberately transmits downbits to the serving base station throughout the duration that thisunfavorable channel condition exists. The base station decodes the fullC/I value on the re-synch subchannel and determines that the full C/Ivalue is equal to the threshold value, which corresponds to the minimumvalue of the dynamic range. If the base station then receives any downbits, the base station is programmed to refrain from updating theregisters that store the current channel conditions until another fullC/I value is received that does not match the threshold value. However,if the base station receives up bits, then the base station updates theregisters accordingly.

[0085]FIG. 7 illustrates the benefit of these embodiments. A fadingcurve is shown against a threshold value X dB. If the fade dips belowthe threshold, then the remote station transmits the representation ofthe threshold value X dB on the re-synch subchannel and down bits on thedifferential feedback subchannel. If the down bits where taken intoaccount, then a situation arises wherein up bits could be transmittedbefore the transmission of the full C/I value on the re-synchsubchannel. The estimate of the fade would follow line 700 until there-synch message is received at point t_(re-synch). However if the downbits were not taken into account, then the transmission of up bits wouldcommence at point t_(up). The estimate of the fade would follow line 710until the re-synch message is received at point t_(re-synch). As one mayobserve, line 710 is a better approximation of the fading condition thanline 700. Hence, implementation of this embodiment improves the abilityof the base station to track the channel conditions.

[0086] The use of a threshold for updating the channel state informationregisters has an additional benefit: the effects of bit errors on thedifferential feedback subchannel are mitigated because the base stationcan be configured to recognize the pattern of constant up bits orconstant down bits on the differential feedback subchannel. In otherwords, if the threshold value is transmitted, and the incremental valuesare constant for the duration that the threshold value is exceeded, thenthe base station will know that an occasional, isolated bit that isdifferent from the expected, constant stream of bits is an error.

[0087] Those of skill in the art would understand that information andsignals may be represented using any of a variety of differenttechnologies and techniques. For example, data, instructions, commands,information, signals, bits, symbols, and chips that may be referencedthroughout the above description may be represented by voltages,currents, electromagnetic waves, magnetic fields or particles, opticalfields or particles, or any combination thereof.

[0088] Those of skill would further appreciate that the variousillustrative logical blocks, modules, circuits, and algorithm stepsdescribed in connection with the embodiments disclosed herein may beimplemented as electronic hardware, computer software, or combinationsof both. To clearly illustrate this interchangeability of hardware andsoftware, various illustrative components, blocks, modules, circuits,and steps have been described above generally in terms of theirfunctionality. Whether such functionality is implemented as hardware orsoftware depends upon the particular application and design constraintsimposed on the overall system. Skilled artisans may implement thedescribed functionality in varying ways for each particular application,but such implementation decisions should not be interpreted as causing adeparture from the scope of the present invention.

[0089] The various illustrative logical blocks, modules, and circuitsdescribed in connection with the embodiments disclosed herein may beimplemented or performed with a general purpose processor, a digitalsignal processor (DSP), an application specific integrated circuit(ASIC), a field programmable gate array (FPGA) or other programmablelogic device, discrete gate or transistor logic, discrete hardwarecomponents, or any combination thereof designed to perform the functionsdescribed herein. A general purpose processor may be a microprocessor,but in the alternative, the processor may be any conventional processor,controller, microcontroller, or state machine. A processor may also beimplemented as a combination of computing devices, e.g., a combinationof a DSP and a microprocessor, a plurality of microprocessors, one ormore microprocessors in conjunction with a DSP core, or any other suchconfiguration.

[0090] The steps of a method or algorithm described in connection withthe embodiments disclosed herein may be embodied directly in hardware,in a software module executed by a processor, or in a combination of thetwo. A software module may reside in RAM memory, flash memory, ROMmemory, EPROM memory, EEPROM memory, registers, hard disk, a removabledisk, a CD-ROM, or any other form of storage medium known in the art. Anexemplary storage medium is coupled to the processor such the processorcan read information from, and write information to, the storage medium.In the alternative, the storage medium may be integral to the processor.The processor and the storage medium may reside in an ASIC. The ASIC mayreside in a user terminal. In the alternative, the processor and thestorage medium may reside as discrete components in a user terminal.

[0091] The previous description of the disclosed embodiments is providedto enable any person skilled in the art to make or use the presentinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

What is claimed is:
 1. In a wireless communication system, an apparatus for scheduling forward link transmissions, comprising: a memory element; and a processing element configured to execute a set of instructions stored on the memory element, the set of instructions for: receiving a full channel quality value and a plurality of incremental channel quality values from a remote station, wherein the plurality of incremental channel quality values are received sequentially; and selectively updating a register with a channel quality estimate, wherein the channel quality estimate is based upon the full channel quality value and the plurality of incremental channel quality values.
 2. The apparatus of claim 1, wherein selectively updating the register with the channel quality estimate comprises: sequentially adding the plurality of incremental channel quality values to the contents of the register; and resetting the register with the full channel quality value when the full channel value is received.
 3. The apparatus of claim 1, wherein the full channel value is received concurrently with more than one incremental channel quality value.
 4. The apparatus of claim 3, wherein selectively updating the register with the channel quality estimate comprises: sequentially adding the plurality of incremental channel quality values to the contents of the register; resetting the register with the full channel quality value when the full channel value is received; summing a portion of the plurality of incremental channel quality values; and adding the summed portion of the plurality of incremental channel quality values to the full channel quality value set in the register.
 5. A method for estimating forward link channel quality from a full channel quality value and a plurality of incremental channel quality values, comprising: decoding the full channel quality value over a plurality of slots; incrementally updating a channel state register with the plurality of incremental channel quality values, wherein each of the plurality of incremental channel quality values are received separately over each of the plurality of slots; and resetting the channel state register with the full channel quality value when the full channel quality value is fully decoded.
 6. The method of claim 5, further comprising: summing a portion of the plurality of incremental channel quality values; and adding the summed portion of the plurality of incremental channel quality values to the full channel quality value stored in the channel state register.
 7. The method of claim 5, further comprising: summing the plurality of incremental channel quality values; and adding the summed plurality of incremental channel quality values to the full channel quality value stored in the channel state register.
 8. The method of claim 5, wherein if the full channel quality value cannot be decoded, then using the plurality of incremental channel quality values as the forward link channel quality.
 9. An apparatus for transmitting channel quality values over a feedback channel to a base station, comprising: a re-synch subchannel generation system for generating full channel quality values; and a differential feedback subchannel generation system for generating a plurality of incremental values, wherein the plurality of incremental values are multiplexed with the full channel quality values.
 10. The apparatus of claim 9, wherein the plurality of incremental values are code-multiplexed with the full channel quality values.
 11. The apparatus of claim 9, wherein the plurality of incremental values are time-multiplexed with the full channel quality values.
 12. The apparatus of claim 9, further comprising a transition indicator subchannel generation system for generating a flag that indicates the start of a transitional period.
 13. The apparatus of claim 12, wherein a Walsh spreading element is used in the re-synch subchannel generation system and not used in the differential feedback subchannel.
 14. The apparatus of claim 12, wherein a common Walsh function is used in the differential feedback subchannel generation system and the transition indicator subchannel generation system.
 15. The apparatus of claim 14, wherein the common Walsh function is used to indicate a base station index.
 16. A method for transmitting channel information from a remote station to a base station, comprising: generating a full channel quality value; and generating an incremental channel quality value, wherein the incremental channel quality value is multiplexed with the full channel quality value.
 17. The method of claim 16, wherein the full channel quality value is generated over more than one slot.
 18. The method of claim 16, wherein the incremental channel quality value is generated over each slot in a channel frame.
 19. The method of claim 18, further comprising: generating a transition indicator, wherein the transition indicator is multiplexed with the incremental channel quality value and the full channel quality value and is used to indicate a transition period for the base station.
 20. Apparatus for estimating forward link channel quality from a full channel quality value and a plurality of incremental channel quality values, comprising: means for decoding the full channel quality value over a plurality of slots; and means for incrementally updating a channel state register with the plurality of incremental channel quality values, wherein each of the plurality of incremental channel quality values are received separately over each of the plurality of slots and for resetting the channel state register with the full channel quality value when the full channel quality value is fully decoded.
 21. Apparatus for transmitting channel information from a remote station to a base station, comprising: means for generating a full channel quality value; and means for generating an incremental channel quality value, wherein the incremental channel quality value is multiplexed with the full channel quality value.
 22. The apparatus of claim 21, further comprising: means for generating a transition indicator, wherein the transition indicator is multiplexed with the incremental channel quality value and the full channel quality value and is used to indicate a transition period for the base station. 